Assessment of soil erosion in the Autonomous District of Abidjan, Cote d'Ivoire
Abstract
In a context of global changes and climatic uncertainties, this study poses the problem of the occurrence of water erosion induced by natural constraints and uncontrolled and speculative anthropisation of soils in the Autonomous District of Abidjan. The aim is to propose a decision-making tool that identifies the areas of the Autonomous District of Abidjan (ADA) most prone to soil loss and the risk of water erosion. More specifically, the aim is to evaluate, prioritise and map the levels of susceptibility to the occurrence of this hydroclimatic risk in the ADA. The mapping approach is based on the aggregation of factors such as climatic aggressiveness, soil erodibility, topography, soil cover and anti-erosion practices from the Revised Universal Soil Loss Equation (RUSLE). The exploitation of these data reveals that soil loss in the ADA oscillates between a minimum value of 0 t/ha/year largely observable in the south of the lagoon system to a maximum value of 107.53 t/ha/year which concerns the plateau areas in the north. The average of this series of 0.48 t/ha/year translates into an overall low level of water erosion in the ADA over nearly 95% of the ADA territory. This soil degradation, which impacts human settlements induced by the morphology of the site, is amplified by human activities. In a context of rapid urban expansion linked to a remarkable demographic growth coupled with climatic uncertainties, development policies must better integrate sustainable soil management.
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Introduction
In sub-Saharan Africa, water erosion is a major risk with more than 20% of land degraded, affecting more than 65% of the population (FAO, 2015). For Chafai and al., (2022), this phenomenon appears as a warning signal of the imbalance between the soil environment and its exploitation system. This risk can affect infrastructure such as roads, riverbanks, crop areas and urban land (Morgan, 2005; Sourlamtas, 2019). A direct product of complex interactions between natural and anthropogenic factors (Phinzi and Ngetar, 2019), it impacts on food production, drinking water quality, ecosystem services, landslides, eutrophication, biodiversity and declining carbon stocks (Boardman and Poesen, 2006; Panagos, 2015). This is a triptych due to firstly the extraction of particles from the soil surface, secondly the transport of these particles by water and thirdly the deposition of the soil when there is no more energy to move the particles (Renard and al., 1997; Morgan, 2005; Sourlamtas, 2019). The management of this major risk by restoring degraded soils is essential for the sustainability of territories. With this in mind, in May 2022, Abidjan, the economic capital of Côte d'Ivoire, hosted the fifteenth session of the Conference of the Parties (COP15) of the United Nations Convention to Combat Desertification (UNCCD). Adopted on 17 June 1994 and entered into force on 26 December 1996, this convention has 197 States and Parties. More generally, it aims to combat all forms of land degradation, understood as the reduction or disappearance of land resources as a result of land use, one or more phenomena caused by human activity and settlement patterns. Land degradation is defined as the reduction or loss of land resources as a result of land use, human activity and settlement patterns, such assoil erosion caused by wind and/or water, deterioration of the physical, chemical and biological or economic properties of soils, and the long-term loss of natural vegetation (United Nations, 1994).
This article is inline with this desire for sustainable soil management. It is intended as a decision-making tool for soil restoration and the anticipatory and preventive management of crises inherent in the risk of erosion in the Autonomous District of Abidjan (ADA). The choice of this area is justified not only by its morphology, which predisposes it to this hydroclimatic risk, but also by its pre-eminence in the urban framework of Côte d'Ivoire. Indeed, the ADA is home to around 42% of the urban population of Côte d'Ivoire (INS, 2021) and polarises almost 80% of the economic activities of Côte d'Ivoire (World Bank Group, 2019). This demographic and economic concentration is at the root of a rapid artificialisation of the land at a rate of almost 7% per year (Traoré, 2022). These mostly informal and spontaneous changes expose soils initially protected by plant cover to the kinetic energy of raindrops, to the ablation of particles from the upper horizons and increase the exposure and vulnerability of territorial issues to the risk of water erosion (Sourlamtas, 2019).
Through modelling based on the integration of the main soil loss factors of the Revised Universal Soil Loss Equation (RUSLE) model into a Geographic Information System, this study prioritises the susceptibility of occurrence of water erosion in the ADA. Specifically, it (i) maps the main erosive factors, (ii) maps soil loss susceptibility and (iii) identifies the soils most prone to erosion risk in the ADA.
Conclusion
The objective of this study was to model water erosion in the Autonomous District of Abidjan. The approach integrates the parameters of the RUSLE model into a Geographic Information System. It allowed to discretize and map the levels of erosion risk occurrence susceptibility. On analysis, the ADA has an average erosive potential of 0.48 t/ha/year. The map of erosive potential has enabled the identification of the most vulnerable areas on which to concentrate efforts and direct investments aimed at restoring soils and protecting them against water erosion for preventive management of crisis situations. In a research and development context, this tool is important for land use planning in order to achieve sustainable development objectives. In fact, in semi-urban areas where agriculture and urban areas are intermingled as in the ADA, soil erosion can be a significant problem due to exposed agricultural land and urban surfaces heavily waterproofed by developments. This significant vulnerability of human and material stakes to the risk of water erosion will thus crescendo in view of the development prospects of the ADA.
This model, in spite of some limitations linked in particular to the data, seems to be well adapted in the ADA in order to define the relative influence of the factors which condition water erosion.
